Max-Planck scientists report in the 5th December issue of Science about polymers as templates to structure ceramic-type inorganic structures formed through self-assembly are used as structure-directing agents. The combination of ceramic with polymer-type materials opens up a way to new and interesting materials with fascinating structures.
Biological minerals such as bone, teeth or shells are well known materials of complex form and structure leading to unique properties like high strength and durability. A concept commonly found in nature for the morphogenesis of such materials is the use of organic structures formed through self-assembly as structure directing agents. The final morphology is then determined by the cooperative organization of inorganic and organic molecular species. As an example, the delicate filigree microskeleton of certain sea plants is derived from the assembly of large vesicles at the membrane wall of the cell before mineralization. It is the arrangement of these vesicles at the membrane which provides a patterned template for the inorganic components.
It is a great challenge for material scientists to use this concept also for the patterning of synthetic inorganic materials. To this end since the beginning of the 90s researchers have e.g. used low molecular surfactant aggregates as templates to form structured inorganic pore networks with channel diameters of 1.5-10 nanometers.
Now scientists at the Max Planck Institute for Polymer Research have discovered that also polymeric species of molecular weights up to several tens of thousands can be employed as structure directing agents for the synthesis of mesostructured ceramic-type materials. They used diblock copolymers, which are chain molecules consisting of two chemically distinct blocks covalently attached at their chain ends.
"Nature does very similar things by using e.g. proteins, which are folded structures of long chain molecules consisting of amino acids. Employing these higher molecular weight polymeric species, it now becomes possible, to make the transition from the small to the large mesoscopic regime (up to several tens of nanometer) of silica-type mesostructures," explains Ulrich Wiesner, project leader at the Max Planck Institute for Polymer Research and in charge of this current research project.
"In addition, the combination of inorganic siliceous components with block copolymers might lead to a desirable combination of ceramic- and polymeric-type properties not accessible so far; since the block copolymer chemistry (architecture, chain length, composition, etc.) can be varied substantially, it should be possible to fine-tune the properties of the composite," adds Markus Templin, Ph.D. student at the MPI-P and first author of the paper.
In their work the Polymer Research Group shows that by increasing the fraction of the inorganic precursors with respect to the polymer, mesostructures are obtained that exhibit the same symmetry and long range order as phases formed by block copolymers alone. The length scale of these structures as well as the state of macroscopic alignment are varied using concepts know from the study of polymers. These results suggest a simple, easily controlled pathway for the preparation of various silica-type mesostructures. Since the inorganic material is very rigid and insoluble in organic solvents, these studies also open up the possibility to design inorganic ceramic-type structures like balls, cylinders or lamellae on the nanoscale by simply dissolving the material. Only recently new phases in triblock copolymers including helices or other more complex forms have been discovered. There is thus no limit yet for the imagination of scientists to synthesize new inorganic materials with unprecedented form.
Journal
Science